GMC Troubleshooting Questions? Ask a Mechanic for Answers
First easy thing to do is measure the resistance in ohms from the engine to the frame.
Sounds like a ground strap missing.
Do you have a Digital Multimeter?
Ok, select ohms (resistance), and measure between the engine block (good clean metal), and the battery negative terminal.
Should be at or near zero ohms.
Ok, I'll await for your reply after your tests.